I trace out my center line using a ruler, and scribe it with brass. I use a brass rod like a pencil to leave a line, brass doesn't rub off too easily and shows good visibility.
Next I grind edge up, making passes from the tang to the tip. Yes watch that tip it is very easy to grind off too much too soon. Keep working the grind gradually back to the center line. I aproach the center line but do not grind over it. During the final finishing grinds I work up the center line making it crisp and straight, I start doing this when I reach the 220 grit stage.
As you grind stop frequently, and observe the reflection of of the light. Your grind should be straight and true. Straight and true grinds are best achieved by always keeping the steel moving and by watching how evenly you are pressing against the wheel. Don't push too much. If you are pushing the belt is old. It is best to use good belts during the important grinds for best results.